1. Field of Invention
The present invention relates to the field of design and manufacture of concrete.
2. The Prior Art
The considerable part of concrete in world building practice is used for road construction, and problem of coarse aggregates for concrete is urgent for road construction industry. Crushed granite is the universal coarse aggregate for concrete, but it constitutes only 15% of US production of coarse aggregates. High cost and shortage of crushed granite requires the solutions allowing reduction of this coarse aggregate in road construction. One of these solutions is composite concrete pavement designed according to Portland Cement Association Engineering Bulletin (Thickness Design for Concrete Highway and Street Pavements, Portland Cement Association, EB109P). To reduce the consumption of normal concrete with crushed granite or other hard rock as a coarse aggregate it provides a lean concrete subbase and/or lower layer of modulus of rupture in the range from 150 to 450 psi. The purpose of the composite pavements is in replacing of a part of normal concrete by a subbase or lower layer of cheaper concrete. An increase of flexural strength of the subbase and/or lower layer of composite concrete pavement is very efficient; it permits a reduction of the thickness of the normal concrete surface course. The increase of the value of modulus of rupture of lean concrete of subbase from 150 to 450 psi means an increase in equivalent normal concrete thickness of the composite concrete pavement at least by 15% with the possibility of a corresponding reduction of thickness of the normal concrete surface course. Strength and cost of lean concrete is determined mainly by the coarse aggregate. Local or recycled aggregates are used usually for lean concrete, resulting in cost saving and conservation of high-quality aggregates.
The use of cheap small grains coarse aggregates is the one of the ways of obtaining of lean and not only lean concrete. Small grains crushed limestone is one of the cheapest aggregates. According to the US Geological Survey, crushed limestone constitutes 71% of total weight of coarse aggregates for concrete produced in USA. American building practice rejects the use of material finer than 9.5 mm and especially finer than 4.75 mm as a coarse aggregate, though the most popular sizes of coarse aggregate No. 57 and No. 67 according to ASTM C36 can include up to 15 and 10 percents of material finer than 9.5 mm and 4.75 mm, respectively. According to the Specification of Florida DOT (Section 346, Portland Cement Concrete), it is necessary to “produce all concrete using Size No. 57 or Size No. 67 coarse aggregate”. The nominal dimensions of these Sizes are in the range from 25.0 to 4.75 mm and from 19.0 to 4.75 mm, respectively. Grading of these sizes of coarse aggregate can include up to 15 and 10 percents of material finer than 9.5 mm and 4.75 mm, respectively. However these most popular in American building practice Sizes No. 57 and 67 of coarse aggregate for concrete are produced usually without content of material finer than 9.5 mm; material of grading (minus-⅜″) is considered as a primary fines, i. e. as a by-product of manufacture of coarse aggregates of grading required by consumer.
The stockpiling technology of limestone fines is presented in the report of the University of Florida “Research and Techno-Economical Evaluation: Uses of Limestone Byproducts”. According to this report “primary fines (minus-⅜ inch) originate during primary crushing and sizing/washing of aggregate raw material prior to processing by the commercial products plant. These materials are commonly discarded as waste, while the plus-⅜″ material is further crushed and sized/washed to produce commercial coarse aggregate products. Byproduct fines produced during this latter stage of processing are termed secondary fines, and are either discarded as waste, or further processed into fines products. For the purpose of this study, two size fractions of both primary and secondary fines were examined, the coarse fraction (minus ⅜″ plus -200 mesh) including screening (minus-4 mesh, i. e. finer than 4.75 mm, by plus -40 mesh), and the fine fraction (minus-200 mesh).”
According to the above report, “the accumulation of fines (minus-⅜ inch) produced by the coarse aggregate industry in the state of Florida is one of the major problems facing the industry today. According to a survey by the U.S. Bureau of Mines Mineral Industry, plant waste factors for all of the types of fines range from 15% to 25% of total production, a value likely underestimated for the coarse aggregate industry in the state of Florida. The survey also estimates that “there are presently 4 billion tons of quarry fines stockpiled in the United States. These quantities are likely to increase by another two billion tons by the turn of the century in response to increased production levels, stricter environmental regulations, and an increased demands for clean coarse aggregate products.” While discussing of this question nationwide, it should be taken into account that 70% of coarse aggregate in the U.S. building practice is crushed limestone, and about half of stockpiled limestone fines is screenings.
The cost of limestone screenings is less than half that of crushed limestone of regular sizes and less than one-fourth that of crushed granite of regular sizes ($ 3, 6-7, and 12-14 for short ton, respectively, as applied to Midwest and South). It relates to fresh by-products of manufacture of crushed limestone of regular sizes. As applied to stockpiled limestone fines, this difference should be considerably greater.
The object of design of composite concrete pavements is to obtain the highest concrete strength of subbase and lower layer of this pavement with the cheapest coarse aggregate and the moderate consumption of cement.